1. Field
The present disclosure relates a redox flow battery. More specifically, the present disclosure relates to a redox flow battery having a low internal resistance and improved energy efficiency.
2. Description of the Related Art
A redox flow battery is similar to other secondary batteries in that it converts electrical energy, which is put into the battery through a charging process, to chemical energy, stores the chemical energy, and converts the stored chemical energy to electrical energy in a discharging process to provide electrical energy. However, a redox flow battery is different from such a secondary battery in that a redox flow battery uses a tank for storage of an electrode active material since the electrode active material having energy is in a liquid form.
Specifically, in a redox flow battery, a catholyte and an anolyte play the role of an electrode active material. The catholyte and the anolyte are respectively stored in tanks as a solution including a redox transition metal which may change its oxidation state.
On the other hand, as in a fuel cell, a cell of the redox flow battery generating electrical energy has a cathode/ion exchange membrane/anode structure. A catholyte and an anolyte are provided from each tank by a pump, and contact a cathode and an anode, respectively. At each contact surface, transition metal ions included in each of the electrolyte solution are oxidized or reduced, thus generating electromotive force corresponding to the Gibbs free energy of the oxidization or the reduction.
In a redox flow battery, an ion exchange membrane does not participate in a reaction but performs the function of rapidly transferring ions, which are charge carriers, between a catholyte and an anolyte. Thus, the ion exchange membrane electrically isolates a cathode and an anode by preventing direct contact between the two electrodes, and inhibits crossover of electrolyte active ions, which are dissolved in the catholyte and the anolyte and which directly participate in a reaction.
In an available redox flow battery, each component is prepared from a material similar to the material of a fuel cell, thus keeping the battery cost high. A significant factor of the battery cost stems from use of Nafion®, which is a fluorinated ion exchange resin membrane, and an electrode which includes carbon felt and a carbon bipolar plate. Carbon felt, which is exclusively imported from other countries to Korea, can lack a surface treatment. On the other hand, a carbon bipolar plate is often cut, and the cutting process adds cost. Also, as a result, the thickness of a battery may not be minimized. A cation exchange membrane, such as Nafion®, has a low ion conductivity because a large size cation such as a quaternary ammonium cation (N(CH3CH2)4+) acts as a charge conductor and has a low charge/discharge efficiency (Coulomb efficiency) due to a severe crossover of ions. Therefore, the overall energy efficiency of the battery may be low.
Thus, there remains a need in a more cost efficient redox flow battery having high energy capacity.